A Quick Guide to Annealing: What is Annealed Metal?
Annealing is a specific process of heat treatment that alters the properties of metal. While there are many different types of heat treatment, annealing is popular because it increases ductability and reduces hardness. In this post, we’ll cover everything you need to know about the process of annealing and annealed metal.
What Is The Annealing Process?
Annealing is a heat treatment process that is common in manufacturing because it improves the physical and sometimes chemical properties of metal to be both more durable and more workable. When heated during the specific process of annealing, atoms migrate in their crystal lattice and the number of atom dislocations goes down, leading to changes in both ductility and hardness. As the material cools it crystallizes again.
For many alloys, including the most common in manufacturing, carbon steel, the properties of the metal are determined by the size of the crystal grains and the phase composition. Both change during heating and cooling. With knowledge of the crystal grain composition and the phase diagram, annealing as a heat treatment can be used to take the metal from hard to soft, brittle to ductile. As a result, the metal will be more formable, an obviously favorable property in manufacturing.
What is the Benefit to Annealed Metal?
As already shared, annealing is used to make metal more ductile and less brittle. Here are the three main benefits to annealing:
Annealing makes metals more formable. When metal is stronger and more ductile, it gives manufacturers more leeway in the fabrication process. There is less risk of material fracturing from bending or pressing.
Annealing can also improve a metal’s ability to be machined and improve the lifespans of tools. Hard, brittle metals can cause wear to shop tools. Annealing metals reduces wear and the chance of damage to tools.
Annealing removes what’s called residual stress. Residual stress is what remains in a metal after the original cause of the stress has been removed. For example, residual stress from roll forming could cause a structural to gape when cut with a band-saw. Residual stress can complicate future processes and annealing is a great way to remove it.
What Metals Are Commonly Annealed?
Most commonly, many types of steel and cast iron are annealed in the manufacturing industry. There are also specific types of aluminum, copper, and brass that can be annealed. While steel is generally cooled to room temperature in still air, copper and brass can also be quenched in water.
What Are The Steps in The Annealing Process
There are three main steps in the annealing process:
Metal is composed of a lattice of crystal structures that are known as grains. Sometimes, the structure of the grains themselves cause stress to the metal. During the first phase of the annealing process, called recovery, a furnace or other type of heat source is utilized to raise the temperature of the material to a point that removes internal stresses.
During recrystallization, further heating raises the temperature of the metal to just below its melting point, high enough that the atoms recrystallize, and low enough that it doesn’t melt.
During the grain growth stage, new crystal grains become fully developed as the metal cools that don’t have the metal’s original stress. The final composition–including the ductility and hardness–is determined by the rate of cooling. Once the metal is annealed, there may be final processing like shaping, stamping, or forming.
When Do You Find Annealed Metals Most Often Used?
Most frequently, you’ll find annealed metals in:
Sheet metals, such as cold rolled steel sheet and galvanized steel sheet, are annealed because the process of cold rolling creates too much hardness for further processing. Annealing restores their ductility and formability, which allows for further bending, punching, forming/stretching or cutting without cracking or losing dimensional stability.
Cold-finished bar and metal wire are often annealed because the process of drawing them through a die causes stresses in their grains. This increase in the strength and brittleness of the metal is called work hardening. Annealing removes this work hardening stress, which can facilitate additional drawing steps or allow for specific mechanical properties in the finished steel. Commonly specified half-hard and dead-soft conditions are a result of controlling the final mechanical properties through the annealing process.
Aluminum is commonly annealed to allow for extreme forming and drawing operations that would otherwise cause cracking or tearing in the metal. Deeply drawn parts are often specified as fully annealed, 0 temper or dead soft product.
Annealing is also used to make materials more uniform if welding has led to residual stress in the heat affected zone.
How Does Annealing Compare to Normalizing?
Unlike annealing, normalizing is the process of increasing hardness. To normalize the metal, you increase the temperature of the material above the austenitic range and then cool it in room temperature air. Austenization means to heat the metal to a temperature at which its crystal structure changes from ferrite to austenite. If you’re looking for a softer, more ductile metal, choose annealing. If you’re looking for a harder, less ductile metal, choose normalizing. Either way, both heat treatments lead to metals with less stress and more machinability.